Do Leptocephali of the European Eel Swim to Reach Continental Waters? Status of the Question

Abstract

We examined recent arguments that leptocephali of the European eel,Anguilla anguilla, swim in an oriented manner, rather than drift, to reach the continental shelf of Europe and the Mediterranean Sea. There is a cline of increasing body length of leptocephali from south to north and from west to east from the Mid-Atlantic Ridge (30°W) to the continental shelf, which could represent migration from south-west to north-east, transport eastward at all latitudes, or increased growth rate with latitude. Evidence that this cline is a trend in age of arriving glass eels along the European coast, and that the duration of migration is less than one year, is weak. Ages reported in the literature for specimens from Morocco to The Netherlands were based on the unvalidated assumption that rings in otoliths were deposited daily. The assumption is unwarranted because of low metabolic rate and uncertainty of nutritional mode of leptocephali. If the assumption were accepted, calculated hatching dates of eels arriving at the European coast imply year-round spawning. Lengths of leptocephali in the Sargasso Sea at various times imply that eels spawn only in late winter and spring. Leptocephali contain tiny amounts of muscle, especially aerobic muscle for sustained swimming. They probably have insufficient capability to swim across the Atlantic in the less than 1–2 y reported by others.